LTC3535 Datasheet by Analog Devices Inc.

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LTC3535
1
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TYPICAL APPLICATION
DESCRIPTION
Dual Channel 550mA 1MHz
Synchronous Step-Up
DC/DC Converter
The LTC
®
3535 is a dual channel, synchronous, fixed fre-
quency step-up DC/DC converter with output disconnect.
Extended battery life in single AA/AAA powered products
is realized with a 680mV start-up voltage and operation
down to 500mV once started.
A switching frequency of 1MHz minimizes solution foot-
print by allowing the use of tiny, low profile inductors
and ceramic capacitors. The current mode PWM design
is internally compensated, reducing external parts count.
The LTC3535 features Burst Mode operation at light load
conditions allowing it to maintain high efficiency over a
wide range of load. Anti-ring circuitry reduces EMI by
damping the inductor in discontinuous mode. Additional
features include a low shutdown current of under 1µA and
thermal shutdown.
The LTC3535 is housed in a 3mm × 3mm × 0.75mm
DFN package.
Efficiency vs Load Current
FEATURES
APPLICATIONS
n Medical Instruments
n Noise Canceling Headphones
n Energy Harvesting
n Bluetooth Headsets
n Two Independent Step-Up Converters
n Each Channel Delivers 3.3V at 100mA from a Single
Alkaline/NiMH Cell or 3.3V at 200mA from Two Cells
n VIN Start-Up Voltage: 680mV
n 1.5V to 5.25V VOUT Range
n Up to 94% Efficiency
n Output Disconnect
n 1MHz Fixed Frequency Operation
n VIN > VOUT Operation
n Integrated Soft-Start
n Current Mode Control with Internal Compensation
n Burst Mode
®
Operation with 9µA IQ Each Channel
n Internal Synchronous Rectifier
n Logic Controlled Shutdown (IQ < 1µA)
n Anti-Ring Control
n Low Profile (3mm × 3mm × 0.75mm)
12-Lead DFN Package
LOAD CURRENT (mA)
0.01
EFFICIENCY (%)
50
70
80
90
1000
3535 TA01b
20
30
40
60
0
10
0.1 1 10 100
100
VOUT = 3.3V
VIN = 1.2V
VOUT = 1.8V
3535 TA01
OFF ON
OFF ON
4.7µH
VIN
0.8V
TO 1.5V
VOUT1
1.8V
100mA
VOUT2
3.3V
50mA
10µF
10µF
511k
2.2µF
4.7µH
1M
1.78M
1M
SW1
VIN1
LTC3535
VOUT1
VOUT2
FB1
FB2
SW2GND GND
SHDN1
VIN2
SHDN2
L, LT, LTC, LTM, Linear Technology, Burst Mode and the Linear logo are registered trademarks
of Linear Technology Corporation. All other trademarks are the property of their respective
owners.
LTC3535 i1 \r7777 5 ‘ 1 a; 31 1 ‘ LII, 1 1 E9 31 ‘ 1 L3, 753‘ ‘77 \ [8 ,,J r: L1 L7HflE/EQ
LTC3535
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PIN CONFIGURATIONABSOLUTE MAXIMUM RATINGS
VIN1, VIN2 Voltage ......................................... –0.3V to 6V
SW1, SW2 Voltage
DC ............................................................ –0.3V to 6V
Pulsed <100ns ......................................... –0.3V to 7V
SHDN1, SHDN2, FB1, FB2 Voltage ............... –0.3V to 6V
VOUT1, VOUT2 ................................................ –0.3V to 6V
Operating Temperature Range
(Notes 2, 5) .............................................. –40°C to 85°C
Junction Temperature ........................................... 125°C
Storage Temperature Range ................... –65°C to 150°C
(Note 1)
TOP VIEW
DD PACKAGE
12-LEAD (3mm × 3mm) PLASTIC DFN
12
11
8
9
10
4
5
3
2
1FB1
SHDN1
VIN1
FB2
SHDN2
VIN2
VOUT1
SW1
GND
VOUT2
SW2
GND 67
13
θJA = 43°C/W, θJC(PAD) = 3°C/W,
EXPOSED PAD (PIN 13) IS GND, MUST BE SOLDERED TO PCB
ORDER INFORMATION
LEAD FREE FINISH TAPE AND REEL PART MARKING PACKAGE DESCRIPTION TEMPERATURE RANGE
LTC3535EDD#PBF LTC3535EDD#TRPBF LDWV 12-Lead (3mm × 3mm) Plastic DFN –40°C to 85°C
Consult LTC Marketing for parts specified with wider operating temperature ranges.
Consult LTC Marketing for information on non-standard lead based finish parts.
For more information on lead free part marking, go to: http://www.linear.com/leadfree/
For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/
(For each channel) The l denotes the specifications which apply over the specified
operating temperature range of –40°C to 85°C, otherwise specifications are at TA = 25°C. VIN = 1.2V, VOUT = 3.3V unless otherwise noted.
PARAMETER CONDITIONS MIN TYP MAX UNITS
Minimum Start-Up Input Voltage ILOAD = 1mA 0.68 0.8 V
Input Voltage Range After Start-Up. (Minimum Voltage is Load Dependent) l0.5 5 V
Output Voltage Adjust Range l1.5 5.25 V
Feedback Pin Voltage l1.165 1.195 1.225 V
Feedback Pin Input Current VFB = 1.30V 1 50 nA
Quiescent Current—Shutdown VSHDN= 0V, Not Including Switch Leakage, VOUT = 0V 0.01 1 µA
Quiescent Current—Active Measured on VOUT, Non-Switching 250 500 µA
Quiescent Current—Burst Measured on VOUT, FB > 1.230V 9 18 µA
N-Channel MOSFET Switch Leakage Current VSW = 5V 0.1 5 µA
P-Channel MOSFET Switch Leakage Current VSW = 5V, VOUT = 0V 0.1 10 µA
N-Channel MOSFET Switch On Resistance VOUT = 3.3V 0.4 Ω
P-Channel MOSFET Switch On Resistance VOUT = 3.3V 0.6 Ω
N-Channel MOSFET Current Limit l550 750 mA
Current Limit Delay to Output (Note 3) 60 ns
Maximum Duty Cycle VFB = 1.15V l87 90 %
ELECTRICAL CHARACTERISTICS
LTC3535 L7 LJUW 3
LTC3535
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Note 1: Stresses beyond those listed under Absolute Maximum Ratings
may cause permanent damage to the device. Exposure to any Absolute
Maximum Rating condition for extended periods may affect device
reliability and lifetime.
Note 2: The LTC3535 is guaranteed to meet performance specifications
from 0°C to 85°C. Specifications over the –40°C to 85°C operating
temperature range are assured by design, characterization and correlation
with statistical process controls.
Note 3: Specification is guaranteed by design and not 100% tested in
production.
(For each channel) The l denotes the specifications which apply over the specified
operating temperature range of –40°C to 85°C, otherwise specifications are at TA = 25°C. VIN = 1.2V, VOUT = 3.3V unless otherwise noted.
ELECTRICAL CHARACTERISTICS
PARAMETER CONDITIONS MIN TYP MAX UNITS
Minimum Duty Cycle VFB = 1.3V l0 %
Switching Frequency l0.75 1 1.25 MHz
SHDN Pin Input High Voltage 0.8 V
SHDN Pin Input Low Voltage 0.3 V
Note 4: Current measurements are made when the output is not switching.
Note 5: This IC includes overtemperature protection that is intended
to protect the device during momentary overload conditions. Junction
temperature will exceed 125°C when overtemperature protection is active.
Continuous operation above the specified maximum operating junction
temperature may result in device degradation or failure.
Note 6: Failure to solder the exposed backside of the package to the PC
board ground plane will result in a thermal resistance much higher than
43°C/W.
TYPICAL PERFORMANCE CHARACTERISTICS
(Each Channel) TA = 25°C, unless otherwise noted.
Efficiency vs Load Current
and VIN for VOUT = 1.8V
Efficiency vs Load Current
and VIN for VOUT = 3.3V
No-Load Input Current vs VIN
LOAD CURRENT (mA)
0.01
40
EFFICIENCY (%)
POWER LOSS (mW)
50
60
70
80
0.1 1 10 100 1000
3535 G01
30
20
10
0
90
100
1
10
100
0.1
0.01
1000
VIN = 1.0V
VIN = 1.2V
VIN = 1.5V
POWER LOSS
EFFICIENCY
LOAD CURRENT (mA)
0.01
40
EFFICIENCY (%)
POWER LOSS (mW)
50
60
70
80
0.1 1 10 100 1000
3535 G02
30
20
10
0
90
100
1
10
100
0.1
0.01
1000
VIN = 1.2V
VIN = 1.8V
VIN = 2.4V
VIN = 3.0V
POWER LOSS
EFFICIENCY
VIN (V)
0.5
IIN (µA)
60
70
80
4.5
3535 G04
50
40
10
1.5 2.5 3.5
1.0 2.0 3.0 4.0
30
20
100
90
VOUT = 3.3V
VOUT = 5V
VOUT = 2.5V
VOUT = 1.8V
LTC3535 mm AVE BURST ENTER BURST T LEAVE EURSTi /—\ ENTER BURSTr ENTER BURS \
LTC3535
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VIN (V)
1.0
0
DELAY (µs)
10
30
40
50
100
70
2.0 3.0 3.5
3535 G07
20
80
90
60
1.5 2.5 4.0 4.5
VIN (V)
1
0
LOAD CURRENT (mA)
5
10
15
20
25
30
1.25 1.5
3535 G08a
ENTER BURST
LEAVE BURST
VOUT = 1.8V
L = 4.7µH
VIN (V)
1
LOAD CURRENT (mA)
15
20
25
3535 G08b
10
5
01.25 1.5 1.75
30
35
40
2
ENTER BURST
LEAVE BURST
VOUT = 2.5V
L = 4.7µH
VIN (V)
1.0
0
LOAD CURRENT (mA)
5
15
20
25
40
35
1.5 2.0
3535 G08c
10
30
2.5 3.0
ENTER BURST
LEAVE BURST
VOUT = 3.3V
L = 4.7µH
VIN (V)
1.0
LOAD CURRENT (mA)
40
50
2.5 3.5
3535 G08d
30
20
1.5 2.0 3.0 4.0 4.5
10
0
ENTER BURST
LEAVE BURST
VOUT = 5V
L = 4.7µH
VOUT (V)
1.5
FREQUENCY CHANGE (%)
1
2
3
3.0 4.0
3535 G09
0
–1
2.0 2.5 3.5 4.5 5.0
–2
–3
NORMALIZED TO VOUT = 3.3V
Oscillator Frequency Change
vs VOUT
Start-Up Delay Time vs VIN
Burst Mode Threshold Current
vs VIN
Burst Mode Threshold Current
vs VIN
Burst Mode Threshold Current
vs VIN
Burst Mode Threshold Current
vs VIN
Efficiency vs Load Current
and VIN for VOUT = 5V
Maximum Output Current vs VIN
Minimum Load Resistance
During Start-Up vs VIN
LOAD CURRENT (mA)
0.01
40
EFFICIENCY (%)
POWER LOSS (mW)
50
60
70
80
0.1 1 10 100 1000
3535 G03
30
20
10
0
90
100
1
10
100
0.1
0.01
1000
VIN = 1.2V
VIN = 2.4V
VIN = 3.6V
VIN = 4.2V
POWER LOSS
EFFICIENCY
VIN (V)
0.5
IOUT (mA)
200
300
4.5
3535 G05
100
01.5 2.5 3.5
1.0 2.0 3.0 4.0
400
150
250
50
350
VOUT = 3.3V
VOUT = 5V
VOUT = 2.5V
VOUT = 1.8V
L = 4.7µH
VIN (V)
0.65
LOAD (Ω)
100
1000
10000
1.050.950.75 1.150.85
3526 G06
10
VOUT = 3.3V
TYPICAL PERFORMANCE CHARACTERISTICS
(Each Channel) TA = 25°C, unless otherwise noted.
LTC3535 / / K m Nag {1: L7 LJUW 5
LTC3535
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TYPICAL PERFORMANCE CHARACTERISTICS
(Each Channel) TA = 25°C, unless otherwise noted.
–50 –30 –10 10 30 50 70 90
TEMPERATURE (°C)
NORMALIZED RDS(ON)
1.0
1.1
1.2
3535 G12
0.8
0.7
1.3
0.9
NORMALIZED TO 25°C
RDS(ON) vs VOUT
Oscillator Frequency Change
vs Temperature
RDS(ON) Change vs Temperature
VOUT (V)
1.5
RDS(ON) (Ω)
0.50
0.45
0.80
0.85
0.90
2.5 3.5 4.0
3535 G10
0.35
0.70
0.60
0.40
0.75
0.30
0.65
0.55
2.0 3.0 4.5 5.0
PMOS
NMOS
–50 –30 –10 10 30 50 70 90
TEMPERATURE (°C)
FREQUECNY CHANGE (%)
4
6
8
3535 G11
0
–10
–8
–6
10
2
–2
–4
NORMALIZED TO 25°C
TEMPERATURE (°C)
–50
0.50
VIN (V)
0.55
0.60
0.65
0.70
0.80
25 0 25 50
3526 G14
75 100
0.75
1mA LOAD
NO LOAD
VFB vs Temperature
Start-Up Voltage vs Temperature
Burst Mode Quiescent Current
vs VOUT
TEMPERATURE (°C)
–60
–1.00
CHANGE IN VFB (%)
–0.75
–0.50
–0.25
0
–20 20 60 100
3535 G13
0.25
0.50
–40 0 40 80
NORMALIZED TO 25°C
VOUT (V)
1.5
10.0
9.5
9.0
8.5
8.0
7.5
7.0 3.0 4.0 4.5
3535 G15
2.0 2.5 3.5 5.0
IQ (µA)
MEASURED ON VOUT
Fixed Frequency Switching
Waveform and VOUT Ripple
Burst Mode Waveforms
VOUT and IIN During Soft-Start
SW PIN
2V/DIV
500ns/DIVVIN = 1.2V
VOUT = 3.3V AT 100mA
COUT = 10µF
3535 G16
VOUT
10mV/DIV
AC-COUPLED
SW PIN
2V/DIV
INDUCTOR
CURRENT
0.2A/DIV
10µs/DIVVIN = 1.2V
VOUT = 3.3V
COUT = 10µF
3535 G17
VOUT
20mV/DIV
AC-COUPLED
SHDN PIN
1V/DIV
INPUT
CURRENT
0.2A/DIV
200µs/DIVVOUT = 3.3V
COUT = 10µF
3535 G18
VOUT
1V/DIV
LTC3535 .KNMVWWAAr—HW “fir—1x 6 L7HCU§QB
LTC3535
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TYPICAL PERFORMANCE CHARACTERISTICS
(Each Channel) TA = 25°C, unless otherwise noted.
Load Step Response
(from Burst Mode Operation)
Load Step Response
(Fixed Frequency)
LOAD
CURRENT
50mA/DIV
100µs/DIVVIN = 1.2V
VOUT = 3.3V
50mA TO 100mA STEP
COUT = 10µF
3535 G21
VOUT
100mV/DIV
AC-COUPLED
LOAD
CURRENT
50mA/DIV
50µs/DIVVIN = 1.2V
VOUT = 3.3V
5mA TO 100mA STEP
COUT = 10µF
3535 G22
VOUT
100mV/DIV
AC-COUPLED
Load Step Response
(from Burst Mode Operation)
LOAD
CURRENT
50mA/DIV
100µs/DIVVIN = 3.6V
VOUT = 5V
20mA TO 170mA STEP
COUT = 10µF
3535 G19
VOUT
100mV/DIV
AC-COUPLED
Load Step Response
(Fixed Frequency)
LOAD
CURRENT
50mA/DIV
100µs/DIVVIN = 3.6V
VOUT = 5V
50mA TO 150mA STEP
COUT = 10µF
3535 G20
VOUT
100mV/DIV
AC-COUPLED
LTC3535 L7 LJUW 7
LTC3535
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PIN FUNCTIONS
VOUT1 (Pin 1): Output Voltage Sense and Drain of the
Internal Synchronous Rectifier for Channel 1. PCB trace
length from VOUT1 to the output filter capacitor (4.7µF
minimum) should be as short and wide as possible.
SW1 (Pin 2): Switch Pin for Channel 1. Connect inductor
between SW1 and VIN1. Keep PCB trace lengths as short
and wide as possible to reduce EMI. If the inductor current
falls to zero, or SHDN1 is low, an internal anti-ringing switch
is connected from SW1 to VIN1 to minimize EMI.
GND (Pins 3, 6, Exposed Pad Pin 13): Signal and Power
Ground. Provide a short direct PCB path between GND
and the (–) side of the input and output capacitors. The
exposed pad must be soldered to the PCB ground plane.
It serves as another ground connection and as a means
of conducting heat away from the die.
VOUT2 (Pin 4): Output Voltage Sense and Drain of the
Internal Synchronous Rectifier for Channel 2. PCB trace
length from VOUT2 to the output filter capacitor (4.7µF
minimum) should be as short and wide as possible.
SW2 (Pin 5): Switch Pin for Channel 2. Connect inductor
between SW2 and VIN2. Keep PCB trace lengths as short
and wide as possible to reduce EMI. If the inductor current
falls to zero, or SHDN2 is low, an internal anti-ringing switch
is connected from SW2 to VIN2 to minimize EMI.
VIN2 (Pin 7): Battery Input Voltage for Channel 2. Connect
a minimum of 1µF ceramic decoupling capacitor from this
pin to ground.
SHDN2 (Pin 8): Logic Controlled Shutdown Input for Chan-
nel 2. There is an internal 4MΩ pull-down on this pin.
SHDN = High: Normal operation.
SHDN = Low: Shutdown, quiescent current < 1µA.
FB2 (Pin 9): Feedback Input to the gm Error Amplifier of
Channel 2. Connect resistor divider tap to this pin. The
output voltage can be adjusted from 1.5V to 5.25V by:
VOUT = 1.195V × [1 + (R4/R3)]
VIN1 (Pin 10): Battery Input Voltage for Channel 1. Connect
a minimum of 1µF ceramic decoupling capacitor from this
pin to ground.
SHDN1 (Pin 11): Logic Controlled Shutdown Input for Chan-
nel 1. There is an internal 4MΩ pull-down on this pin.
SHDN = High: Normal operation.
SHDN = Low: Shutdown, quiescent current < 1µA.
FB1 (Pin 12): Feedback Input to the gm Error Amplifier
of Channel 1. Connect resistor divider tap to this pin. The
output voltage can be adjusted from 1.5V to 5.25V by:
VOUT = 1.195V × [1 + (R2/R1)].
LTC3535 8 L7LJ1‘JW
LTC3535
8
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BLOCK DIAGRAM
10
Σ
+
–
GATE DRIVERS
AND
ANTI-CROSS
CONDUCTION
LOGIC
CLK1
IPK
IPK
COMP
SLOPE
COMP
IZERO
COMP
ERROR AMP
SLEEP COMP
IZERO
WAKE
EXPOSED
PAD
+
–
WELL
SWITCH
MODE
CONTROL
UVLO
VREF VREF1
4M
SHDN1
VBEST
START-UP
1MHz
OSC
TSD
THERMAL
SHUTDOWN
SHUTDOWN
ANTI-RING
VSEL
VIN1
2
1
SW1
VOUT
L1
4.7µH
VB
SHUTDOWN
CLAMP
CSS
VREF
VOUT1
12
13
GND
3535 BD
6
GND
3
FB1
R2
COUT1
10µF
VOUT1
1.5V
TO 5.25V
R1
11
CIN
2.2µF
VIN1
0.8V
TO 5V
Σ
+
–
GATE DRIVERS
AND
ANTI-CROSS
CONDUCTION
LOGIC
CLK2
IPK
IPK
COMP
SLOPE
COMP
IZERO
COMP
ERROR AMP
SLEEP COMP
IZERO
WAKE
+
–
WELL
SWITCH
MODE
CONTROL
UVLO
VREF VREF2
4M
VBEST
START-UP
1MHz
OSC
TSD
THERMAL
SHUTDOWN
SHUTDOWN
ANTI-RING
VSEL
VIN2
4
VOUT2
VB
SHUTDOWN
CLAMP
CSS
VREF
VOUT2
9
FB2
R4
COUT2
10µF
VOUT2
1.5V
TO 5.25V
R3
8
CIN2
2.2µF
VIN2
0.8V
TO 5V
+
–
SHDN2
7VIN2
5SW2
L2
4.7µH
+
–
LTC3535 L7 LJUW 9
LTC3535
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OPERATION
The LTC3535 is a dual channel 1MHz synchronous boost
converter housed in a 12-lead 3mm × 3mm DFN package.
Each channel is identical and fully independent. They can
operate from the same source, or from different voltage
sources.
In addition, their output voltages can be tied together
to allow operation of a single output from two different
input sources. However, note that the two channels are
not designed to current share, so if both input voltages
are present either one may be supplying the load.
The following description of operation applies to each
channel. Note that references to VIN or VOUT apply to the
corresponding channel.
With a guaranteed ability to start up and operate from
inputs less than 0.8V, each channel features fixed fre-
quency, current mode PWM control for exceptional line
and load regulation. The current mode architecture with
adaptive slope compensation provides excellent transient
load response, requiring minimal output filtering. Internal
soft-start and internal loop compensation simplifies the
design process while minimizing the number of external
components.
With its low RDS(ON) and low gate charge internal N-channel
MOSFET switch and P-channel MOSFET synchronous
rectifier, the LTC3535 achieves high efficiency over a wide
range of load currents. Burst Mode operation maintains
high efficiency at very light loads, reducing the quiescent
current to just 9µA per channel. Operation can be best
understood by referring to the Block Diagram.
LOW VOLTAGE START-UP
The LTC3535 includes an independent start-up oscillator
designed to start up at an input voltage of 0.68V (typical).
Soft-start and inrush current limiting are provided during
start-up, as well as normal mode.
When either VIN or VOUT for a given channel exceeds 1.3V
typical, the channel enters normal operating mode. When
the output voltage exceeds the input by 0.24V, the channel
powers itself from VOUT instead of VIN. At this point the
internal circuitry has no dependency on the VIN input volt-
age, eliminating the requirement for a large input capacitor.
The input voltage can drop as low as 0.5V. The limiting
factor for the application becomes the availability of the
power source to supply sufficient energy to the output at
low voltages, and maximum duty cycle, which is clamped at
90% typical. Note that at low input voltages, small voltage
drops due to series resistance become critical, and greatly
limit the power delivery capability of the converter.
LOW NOISE FIXED FREQUENCY OPERATION
Soft-Start
The LTC3535 contains internal circuitry to provide soft-
start operation. The soft-start circuitry slowly ramps the
peak inductor current from zero to its peak value of 750mA
(typical) in approximately 0.5ms, allowing start-up into
heavy loads. The soft-start circuitry is reset in the event
of a shutdown command or a thermal shutdown.
Oscillator
An internal oscillator (independent for each channel) sets
the switching frequency to 1MHz.
Shutdown
Shutdown is accomplished by pulling the SHDN pin below
0.3V and enabled by pulling the SHDN pin above 0.8V.
Although SHDN can be driven above VIN or VOUT (up to the
absolute maximum rating) without damage, the LTC3535
has a proprietary test mode that may be engaged if SHDN
is held in the range of 0.5V to 1V higher than the greater
of VIN or VOUT. If the test mode is engaged, normal PWM
switching action is interrupted, which can cause undesir-
able operation in some applications. Therefore, in appli-
cations where SHDN may be driven above VIN, a resistor
divider or other means must be employed to keep the SHDN
voltage below (VIN + 0.4V) to prevent the possibility of
(Refer to Block Diagram)
LTC3535 ‘IO II|——Iw»— L7LJCUEN2
LTC3535
10
3535fa
the test mode being engaged. Refer to Figure 1 for two
possible implementations.
independent of input or output voltage, unless VOUT falls
below 0.7V, in which case the current limit is cut in half.
Zero Current Comparator
The zero current comparator monitors the inductor cur-
rent to the output and shuts off the synchronous rectifier
when this current reduces to approximately 30mA. This
prevents the inductor current from reversing in polarity,
improving efficiency at light loads.
Synchronous Rectifier
To control inrush current and to prevent the inductor
current from running away when VOUT is close to VIN, the
P-channel MOSFET synchronous rectifier is only enabled
when VOUT > (VIN + 0.24V).
Anti-Ringing Control
The anti-ring circuit connects a resistor across the in-
ductor to prevent high frequency ringing on the SW pin
during discontinuous current mode operation. Although
the ringing of the resonant circuit formed by L and CSW
(capacitance on SW pin) is low energy, it can cause EMI
radiation.
Output Disconnect
The LTC3535 is designed to allow true output disconnect
by eliminating body diode conduction of the internal
P-channel MOSFET rectifier. This allows for VOUT to go
to zero volts during shutdown, drawing no current from
the input source. It also allows for inrush current limiting
at turn-on, minimizing surge currents seen by the input
supply. Note that to obtain the advantages of output dis-
connect, there must not be an external Schottky diode
connected between SW and VOUT. The output disconnect
feature also allows VOUT to be pulled high, without any
reverse current into a battery connected to VIN.
Thermal Shutdown
If the die temperature exceeds 160°C, the LTC3535 will
go into thermal shutdown. All switches will be off and
the soft-start capacitor will be discharged. The device
will be enabled again when the die temperature drops by
about 15°C.
OPERATION
(Refer to Block Diagram)
3535 F01
1M
R
R > (VCNTRL/VIN + 0.4) – 1)MΩ
4M
±30%
LTC3535
SHDN
VCNTRL
VCNTRL
1M
ZETEX
ZC2811E
4M
±30%
LTC3535
SHDN
VIN
Figure 1. Recommended Shutdown Circuits When
Driving SHDN Above VIN
Error Amplifier
The positive input of the transconductance error amplifier
is internally connected to the 1.195V reference and the
negative input is connected to FB. Clamps limit the mini-
mum and maximum error amp output voltage for improved
large-signal transient response. Power converter control
loop compensation is provided internally. An external
resistive voltage divider from VOUT to ground programs
the output voltage via FB from 1.5V to 5.25V.
VV
R
R
OUT
=+
1 195 1 2
1
.•
Current Sensing
Lossless current sensing converts the peak current signal of
the N-channel MOSFET switch into a voltage that is summed
with the internal slope compensation. The summed signal
is compared to the error amplifier output to provide a peak
current control command for the PWM.
Current Limit
The current limit comparator shuts off the N-channel
MOSFET switch once its threshold is reached. The cur-
rent limit comparator delay to output is typically 60ns.
Peak switch current is limited to approximately 750mA,
LTC3535 L7HDN§AE 1 1
LTC3535
11
3535fa
Burst Mode OPERATION
Each channel of the LTC3535 will enter Burst Mode
operation at light load current and return to fixed frequency
PWM mode when the load increases. Refer to the Typical
Performance Characteristics to see the output load Burst
Mode threshold current vs VIN. The load current at which
Burst Mode operation is entered can be changed by
adjusting the inductor value. Raising the inductor value
will lower the load current at which Burst Mode operation
is entered.
In Burst Mode operation, the LTC3535 still switches at a
fixed frequency of 1MHz, using the same error amplifier
and loop compensation for peak current mode control.
This control method eliminates any output transient when
switching between modes. In Burst Mode operation,
energy is delivered to the output until it reaches the
nominal regulation value, then the LTC3535 transitions
to sleep mode where the outputs are off and the LTC3535
consumes only 9µA of quiescent current from VOUT for
each channel. When the output voltage droops slightly,
switching resumes. This maximizes efficiency at very light
loads by minimizing switching and quiescent losses. Burst
Mode output voltage ripple, which is typically 1% peak-to-
peak, can be reduced by using more output capacitance
(10µF or greater), or with a small capacitor (10pF to 50pF)
connected between VOUT and FB.
As the load current increases, the LTC3535 will automati-
cally leave Burst Mode operation. Note that larger output
capacitor values may cause this transition to occur at
lighter loads. Once the LTC3535 has left Burst Mode op-
eration and returned to normal operation, it will remain
there until the output load is reduced below the burst
threshold current.
Burst Mode operation is inhibited during start-up and
soft-start and until VOUT is at least 0.24V greater than VIN.
Note that each channel can enter or leave Burst Mode
operation independent of the other channel.
OPERATION
(Refer to Block Diagram)
APPLICATIONS INFORMATION
VIN > VOUT OPERATION
The LTC3535 will maintain voltage regulation even when
the input voltage is above the desired output voltage. Note
that the efficiency is much lower in this mode, and the
maximum output current capability will be less. Refer to
the Typical Performance Characteristics.
SHORT-CIRCUIT PROTECTION
The LTC3535 output disconnect feature allows output
short circuit while maintaining a maximum internally set
current limit. To reduce power dissipation under short-
circuit conditions, the peak switch current limit is reduced
to 400mA (typical per channel).
SCHOTTKY DIODE
Although not recommended, adding a Schottky diode from
SW to VOUT will improve efficiency by about 2%. Note
that this defeats the output disconnect and short-circuit
protection features.
PCB LAYOUT GUIDELINES
The high speed operation of the LTC3535 demands careful
attention to board layout. A careless layout will result in
reduced performance. Figure 2 shows the recommended
component placement. A large ground pin copper area
will help to lower the die temperature. A multilayer board
with a separate ground plane is ideal, but not absolutely
necessary.
Figure 2. Recommended Component Placement
VIN2
VIN1
VOUT2
VOUT1
GNDGND
SHDN
SHDN
LTc3535 Ripp
LTC3535
12
3535fa
APPLICATIONS INFORMATION
COMPONENT SELECTION
Inductor Selection
The LTC3535 can utilize small surface mount chip induc-
tors due to their fast 1MHz switching frequency. Inductor
values between 3.3µH and 6.8µH are suitable for most
applications. Larger values of inductance will allow slightly
greater output current capability (and lower the Burst
Mode threshold) by reducing the inductor ripple current.
Increasing the inductance above 10µH will increase com-
ponent size while providing little improvement in output
current capability.
The minimum inductance value is given by:
LVV V
RippleV
IN MIN OUT MAXINMIN
OUT
>
() ()
(
•MMAX)
where:
Ripple = Allowable inductor current ripple (amps peak-
peak)
VIN(MIN) = Minimum input voltage
VOUT(MAX) = Maximum output voltage
The inductor current ripple is typically set for 20% to
40% of the maximum inductor current. High frequency
ferrite core inductor materials reduce frequency dependent
power losses compared to cheaper powdered iron types,
improving efficiency. The inductor should have low ESR
(series resistance of the windings) to reduce the I2R power
losses, and must be able to support the peak inductor
current without saturating. Molded chokes and some chip
inductors usually do not have enough core area to support
the peak inductor current of 750mA seen on the LTC3535.
To minimize radiated noise, use a shielded inductor. See
Table 1 for suggested components and suppliers.
Table 1. Recommended Inductors
VENDOR PART/STYLE
Coilcraft
(847) 639-6400
www.coilcraft.com
LPO4815
LPS4012, LPS4018
MSS5131
MSS4020
MOS6020
ME3220
DS1605, DO1608
Coiltronics
www.cooperet.com
SD10, SD12, SD14, SD18, SD20,
SD52, SD3114, SD3118
FDK
(408) 432-8331
www.fdk.com
MIP3226D4R7M, MIP3226D3R3M
MIPF2520D4R7
MIPWT3226D3R0
Murata
(714) 852-2001
www.murata.com
LQH43C
LQH32C (-53 series)
301015
Sumida
(847) 956-0666
www.sumida.com
CDRH5D18
CDRH2D14
CDRH3D16
CDRH3D11
CR43
CMD4D06-4R7MC
CMD4D06-3R3MC
Taiyo-Yuden
www.t-yuden.com
NP03SB
NR3015T
NR3012T
TDK
(847) 803-6100
www.component.tdk.com
VLP
VLF, VLCF
Toko
(408) 432-8282
www.tokoam.com
D412C
D518LC
D52LC
D62LCB
Wurth
(201) 785-8800
www.we-online.com
WE-TPC type S, M
Output and Input Capacitor Selection
Low ESR (equivalent series resistance) capacitors should
be used to minimize the output voltage ripple. Multilayer
ceramic capacitors are an excellent choice as they have
extremely low ESR and are available in small footprints. A
LTC3535 «H |~ ”HI—- "H'— «ma L7 LJUW 13
LTC3535
13
3535fa
APPLICATIONS INFORMATION
4.7µF to 10µF output capacitor is sufficient for most ap-
plications. Larger values may be used to obtain extremely
low output voltage ripple and improve transient response.
X5R and X7R dielectric materials are preferred for their
ability to maintain capacitance over wide voltage and
temperature ranges. Y5V types should not be used.
The internal loop compensation of the LTC3535 is designed
to be stable with output capacitor values of 4.7µF or greater
(without the need for any external series resistor). Although
ceramic capacitors are recommended, low ESR tantalum
capacitors may be used as well.
A small ceramic capacitor in parallel with a larger tantalum
capacitor may be used in demanding applications that have
large load transients. Another method of improving the
transient response is to add a small feed-forward capacitor
across the top resistor of the feedback divider (from VOUT
to FB). A typical value of 22pF will generally suffice.
Low ESR input capacitors reduce input switching noise
and reduce the peak current drawn from the battery. It
follows that ceramic capacitors are also a good choice
for input decoupling and should be located as close as
possible to the device. A 2.2µF input capacitor is sufficient
for most applications, although larger values may be
used without limitations. Table 2 shows a list of several
ceramic capacitor manufacturers. Consult the manufactur-
ers directly for detailed information on their selection of
ceramic capacitors.
Table 2. Capacitor Vendor Information
SUPPLIER PHONE WEBSITE
AVX (803) 448-9411 www.avxcorp.com
Murata (714) 852-2001 www.murata.com
Taiyo-Yuden (408) 573-4150 www.t-yuden.com
TDK (847) 803-6100 www.component.tdk.com
Samsung (408) 544-5200 www.sem.samsung.com
TYPICAL APPLICATION
3535 TA02
4.7µH
VIN
0.8V
TO 1.5V
VOUT
3.3V
30mA
VHOLDUP
10µF
CHOLD*
0.47F 1M
4.25V
2.2µF
2.2µF
4.7µH
1.78M
392k
1M
1.5M
499k
SW
VIN1
LTC3535
VOUT1
VOUT2
FB1
FB2
SW2GND GND
SHDN1
VIN2
SHDN2
*POWERSTOR PA-5R0H474-R
+
Single Cell to 3.3V Converter with 20 Seconds of Holdup with 30mA Load
VIN
1V/DIV
VOUT
2V/DIV
5s/DIV 3535 TA02b
VHOLDUP
2V/DIV
LTC3535 14 7*7r7*7 L7LJCUEN2
LTC3535
14
3535fa
PACKAGE DESCRIPTION
DC Package
12-Lead Plastic DFN (3mm × 3mm)
(Reference LTC DWG # 05-08-1725 Rev A)
3.00 ± 0.10
(4 SIDES)
NOTE:
1. DRAWING IS NOT A JEDEC PACKAGE OUTLINE
2. DRAWING NOT TO SCALE
3. ALL DIMENSIONS ARE IN MILLIMETERS
4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE
MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE
5. EXPOSED PAD AND TIE BARS SHALL BE SOLDER PLATED
6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON THE
TOP AND BOTTOM OF PACKAGE
0.40 ± 0.10
BOTTOM VIEW—EXPOSED PAD
1.65 ± 0.10
0.75 ± 0.05
R = 0.115
TYP
16
127
PIN 1
TOP MARK
(SEE NOTE 6)
0.200 REF
0.00 – 0.05
(DD12) DFN 0106 REV A
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED
0.23 ± 0.05
0.25 ± 0.05
2.25 REF
2.38 ± 0.05
1.65 ± 0.05
2.10 ± 0.05
0.70 ± 0.05
3.50 ± 0.05
PACKAGE
OUTLINE
PIN 1 NOTCH
R = 0.20 OR
0.25 × 45°
CHAMFER
2.38 ± 0.10
2.25 REF
0.45 BSC
0.45 BSC
LTC3535 L7HEJWEGR 1 5
LTC3535
15
3535fa
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no representa-
tion that the interconnection of its circuits as described herein will not infringe on existing patent rights.
REVISION HISTORY
REV DATE DESCRIPTION PAGE NUMBER
A 9/10 Updated Applications section
Updated Pin Configuration
Updated Note 6
Updated Pins 3, 6 and 13 text
Updated Shutdown section
Corrected CHOLD capacitor value in Typical Application
Added new Typical Application and Updated Related Parts table
1
2
3
7
9, 10
13
16
LTC3535 ||_{ 'II’ _H_ i—L [Mm-I _I_L % L? T —H— A i i l T % —H— _:}_ 16 L7LJCUEN2
LTC3535
16
3535fa
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com
LINEAR TECHNOLOGY CORPORATION 2009
LT 0910 REV A • PRINTED IN USA
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LTC3527/LTC3527-1 Dual 800mA and 400mA (ISW), 2.2MHz, Synchronous
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LTC3528
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1A (ISW), 1MHz Synchronous Step-Up DC/DC with
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ISD < 1µA, 2mm × 3mm DFN-8 Package
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94% Efficiency VIN: 0.7V to 5V, VOUT(MAX) = 5.25V, IQ = 30µA,
ISD < 1µA, 3mm × 3mm QFN-16 Package
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LTC3539-2
2A (ISW), 1/2MHz, Synchronous Step-Up DC/DC
Converter with Output Disconnect
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3535 TA03
4.7µH
COILCRAFT XFL4020-472
COILCRAFT XFL4020-472
SINGLE
AAA CELL 3.3V VOUT
10µF
2.2µF
4.7µH
30.1k
1M
1.78M
SW1
VIN1
LTC3535
VOUT1
VOUT2
FB1
FB2
SW2GND GND
SHDN1
VIN2
SHDN2
+
47µF
1nF
330pF
T1
1:100
COILCRAFT
LPR6235-752SML
TEG
FERROTEC 9500/127/100B
1µF
LTC3108
VSTORE
VOUT2_EN
2.35V PEAK
VOUT
VOUT2
VLDO
PGD
C1
C2
SW
VS1
VS2
VAUX
GND
2.2µF
C1
1.5mF
1µF
D1A
D1B
BAT54C
••
+
3.3V Converter Operates from a Single Cell or from Harvested Thermal Energy, as Low as 1°C ∆T
TYPICAL APPLICATION